File Locking In Go

filelockingingo.go

package main

import (
	"flockexper/lock"
	"fmt"
	"io/ioutil"
	"os"
	"path/filepath"
	"sync"
	"time"
)

var wg sync.WaitGroup

func main() {
	wg.Add(2)
	go func() {
		if err := readerTask("/tmp/test"); err != nil {
			fmt.Println(err.Error())
		}
	}()
	go func() {
		if err := updateTask("/tmp/test"); err != nil {
			fmt.Println(err.Error())
		}
	}()

	wg.Wait()
}

func updateTask(dirName string) error {
	defer wg.Done()
	
	locker := lock.NewExclusiveLock(dirName)
	if err := locker.TakeLock(true); err != nil {
		return err
	}
	defer locker.Close()

	dirEntries, err := ioutil.ReadDir(dirName)
	if err != nil {
		return fmt.Errorf("failed to read dir: %w", err)
	}
	for _, dirEntry := range dirEntries {
		fmt.Println("== " + dirEntry.Name())
		srcSubDirName := filepath.Join(dirName, dirEntry.Name())

		if dirEntry.IsDir() {
			destSubDirName := filepath.Join(dirName, dirEntry.Name()+"_v2")
			fmt.Printf("== Moving %s to %s\n", srcSubDirName, destSubDirName)
			if err := os.Rename(srcSubDirName, destSubDirName); err != nil {
				return err
			}
			continue
		}

		if dirEntry.Name() == "c.d" {
			if err := os.Remove(srcSubDirName); err != nil {
				return nil
			}
		}
	}
	return nil
}

func readerTask(dirName string) error {
	defer wg.Done()
	
	locker := lock.NewSharedLock(dirName)
	if err := locker.TakeLock(true); err != nil {
		return err
	}
	defer locker.Close()

	return read(dirName)
}

func read(dirName string) error {
	dirEntries, err := ioutil.ReadDir(dirName)
	if err != nil {
		return fmt.Errorf("failed to read dir: %w", err)
	}

	for _, dirEntry := range dirEntries {
		filePath := filepath.Join(dirName, dirEntry.Name())
		fmt.Println("-- Reading " + filePath)

		if dirEntry.IsDir() {
			read(filePath)
			continue
		}

		f, err := ioutil.ReadFile(filePath)
		if err != nil {
			return err
		}
		fmt.Println(string(f))

		time.Sleep(1 * time.Second)
	}
	return nil
}

filelockingmanager.go

package lock

import (
	"os"
	"path/filepath"
	"syscall"
)

const _lockPostfix = ".lock"

type Manager struct {
	dirName  string
	fileName string
	lockType int
	file     *os.File
}

func NewExclusiveLock(dirName string) *Manager {
	return &Manager{
		dirName:  dirName,
		fileName: getLockFileName(dirName),
		lockType: syscall.LOCK_EX,
	}
}

func NewSharedLock(dirName string) *Manager {
	return &Manager{
		dirName:  dirName,
		fileName: getLockFileName(dirName),
		lockType: syscall.LOCK_SH,
	}
}

func (m *Manager) TakeLock(waitToAcquire bool) error {
	lockType := syscall.LOCK_EX
	if !waitToAcquire {
		lockType = lockType | syscall.LOCK_NB
	}

	file, err := os.OpenFile(m.fileName, os.O_WRONLY|os.O_CREATE, 0666)
	if err != nil {
		return err
	}

	m.file = file

	return syscall.Flock(int(file.Fd()), lockType)
}

func (m *Manager) Close() error {
	if err := syscall.Flock(int(m.file.Fd()), syscall.LOCK_UN); err != nil {
		return err
	}

	return m.file.Close()
}

func getLockFileName(dirName string) string {
	parentDir := filepath.Dir(dirName)
	base := filepath.Base(dirName)
	return filepath.Join(parentDir, base+_lockPostfix)
}